Problem: In geometry class, the girl to boy ratio is $3$ to $8$. If there are $15$ girls, how many boys are there?
Answer: A ratio of $3$ girls to $8$ boys means that there is a group of $3$ girls for every group of $8$ boys. If there are $15$ girls, then there are $5$ groups of $3$ girls each. Because we know the ratio is $3:8$ , there must be $5$ groups of $8$ boys each. There is a total of $40$ boys in geometry class.